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Looking Glass

published 30/12/2020 in Virtualization | tags : Linux, VFIO, Virtualization, Tutorial, BIOS, LookingGlass, #100DaysToOffload

Estimated read time: 4 min.

WTF is Looking Glass?

LG is at two part application that uses a shared memory device, that allows dumping of a framebuffer through the PCI bus to be rendered by a secondary GPU.

This is a fancy way to say you can have Windows powered by a secondary GPU rendered in your Host OS as either a window or full-screen with very little overhead or latency.


  1. Setup your /dev/shm/looking-glass
  2. Install the client in your Host OS.
  3. Configure your Guest VM to use the shm device

Setting up Looking Glass is very straight forward, and they put together a really great quick-start guide on their homepage. I highly suggest following along with their directions.

However it seems that there’s always a bit of confusion as to how the various components are named and what you should be downloading and installing where.

I’ll briefly describe the steps involved with setting up Looking Glass as some of the pieces are named in a way that sounds more complicated than it actually is.

Setting up /dev/shm/looking-glass

Memory reqs

If your rendering a guest between 1920 x 1080 or 2560 x 1440 you’ll need 32mb otherwise larger resolutions should create a device 64mb in size.


Use systemd to create a your shared memory device, create a file located at /etc/tmpfiles.d/10-looking-glass.conf with the following content:

f    /dev/shm/looking-glass  0660    yourusernamehere    kvm     -

Replace ‘yourusernamehere’ with your own user, this will make sure that your device gets recreated at boot time. Now you can create it right away with the following command, as to not have to reboot right away.

systemd-tmpfiles --create /etc/tmpfiles.d/10-looking-glass.conf

Installing the client on your Host OS

Follow the compiling directions for your distribution and make sure you install the dependencies first. Directions for compiling can be found here in the LG quickstart guide.

Once it’s compiled make sure to grab the binary and fire it in your PATH somewhere’s so you can use it later.

Next up we will want to add the device to your virtual machine’s configuration. This can be done with the following command:

virsh edit win10

Where win10 is your virtual machine configuration name. Add the following block near the bottom above the end </devices> section.

<shmem name='looking-glass'>
  <model type='ivshmem-plain'/>
  <size unit='M'>64</size>
  <address type='pci' domain='0x0000' bus='0x09' slot='0x01' function='0x0'/>

Make sure to swap out the value with the one determined above (32 or 64) in the previous step. Now a Ram Drive will show up in your Windows guest the next time we boot it up and you’ll need to install the VirtIO driver for it.

Configure your Guest VM

Boot up your Guest and install the VirtIO Ram Drive driver, you can find the driver here.

Download and install the Looking Glass “host” Windows application from the looking glass site inside your Guest and make sure it’s the same version as you compiled the “client” for in your Host OS.

Visible Confusion

So looking glass has 2 components:

  • LG_Host application that runs in your Guest VM
  • looking-glass-client that runs in your Host OS

The LG_Host running in your VM will dump the frames to be rendered to the shared memory device, and you can use a the looking-glass-client to render those frames in either the Host OS or another Guest VM depending on what type of setup your running.

Final steps

Finally you will want to change your display type to none from QXL in virt-manager to allow the looking-glass-client to take over responsibility for displaying output from the VM.

video none

Looking glass installs the the host application as a Windows service, so it should be started when the VM boots up.

Demo Video